Borehole surveying tool deployment

ABSTRACT

An apparatus for use in a downhole survey in conjunction with core sampling comprises a body adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the apparatus and operated in the borehole.

FIELD OF THE INVENTION

This invention relates to borehole surveying operations. In particular the invention relates to core sampling in borehole surveying operations.

BACKGROUND ART

The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

There is a need for core sampling in borehole surveying operations.

Core samples are obtained through core drilling operations. Core drilling is typically conducted with a core drill comprising outer and inner tube assemblies. The inner tube assembly comprises a core inner tube. A cutting head is attached to the outer tube assembly so that rotational torque applied to the outer tube assembly is transmitted to the cutting head. A core is generated during the drilling operation, with the core progressively extending along the core inner tube as drilling progresses. When a core sample is required, the core within the core inner tube is fractured. The inner tube assembly and the fractured core sample contained therein are then retrieved from within the drill hole, typically by way of a retrieval cable (which is commonly referred to as a wireline) lowered down the drill hole. Once the inner tube assembly has been brought to ground surface, the core sample can be removed from the core inner tube and subjected to the necessary analysis.

The inner tube assembly further comprises a backend assembly which includes a spearhead point releasably engagable with an overshot attached to the end of the wireline. With this arrangement, the inner tube assembly can be lowered into, and retrieved from, the outer tube assembly and the drill string to which the outer tube assembly is connected.

During a borehole drilling operation there is a need to survey the path of the borehole to determine if the trajectory is being maintained within acceptable limits. Surveying a borehole is usually accomplished using a surveying tool which is moved along the borehole to obtain the information required, or at least data from which the required information can be determined. Information in relation to the path of a borehole can typically include inclination, azimuth and depth.

Surveying tools typically contain sensor devices for measuring the direction and magnitude of the local gravitational field, the Earth's magnetic field and/or the rate of rotation of the Earth. These measurements correspond to the position and orientation of the surveying tool in the borehole. The position, inclination and/or azimuth can be calculated from these measurements.

The sensor devices can comprise accelerometers for measuring the direction and magnitude of the local gravitational field, magnetometers for measuring the Earth's magnetic field and/or gyroscopes for measuring the rate of rotation of the Earth, from which azimuth can be calculated.

Typically, the core drilling operation is performed at an angle to the vertical, and it is desirable for analysis purposes to have an indication of the orientation of the core sample relative to the underground environment from which it was extracted. It is therefore important that there be some means of identifying the orientation the core sample had within the underground environment prior to it having been brought to the surface.

Core orientation devices are used to provide an indication of the orientation of the core sample.

The applicant's International application PCT/AU2011/000628 discloses a down hole surveying system for directional surveying of boreholes. The down hole surveying system is configured as a tool comprising a body which is sized and shaped for movement along a borehole in down hole surveying applications. The body accommodates sensor devices comprising a gyroscope and an accelerometer.

Downhole surveying operations and core retrieval are typically conducted as separate operations. In particular, it is customary to perform a surveying operation once the inner tube assembly has been removed from the drill pipe/rod during retrieval of a core sample. This involves retrieving the inner tube assembly using an overshot attached to the end of the wireline, and then lowering a down hole survey instrument into the borehole to perform the surveying operation. The surveying instrument is then raised and removed from the borehole and the inner tube assembly returned into position for the next core sampling operation.

Consequently, there is downtime for both downhole surveying operations and core retrieval.

In would be advantageous for there to be at least some coordination between downhole surveying and core retrieval operations to thereby reduce downtime while the procedures are being performed.

Also, orientation of a bottom hole assembly and more recently core sample has been performed by using gravity based sensors, namely accelerometers, in angled/inclined drill hole applications. These systems have been able to orientate a pre-determined tool face of a bottom hole assembly down the borehole with a survey instrument by referencing the tool face of the bottom hole assembly to the known top dead centre position as determined by gravity based sensors.

In vertical drilling applications where the inclination of the bottom hole assembly is within 5 degrees of the vertical plane it is not possible to use gravimetric sensors as a reference to top dead centre with any acceptable accuracy. In such applications, magnetic based sensors such as magnetometers can be used to reference the bottom hole assembly tool face to magnetic North but are subject to influence from magnetic sources such as the formation being drilled and the bottom hole assembly itself. The use of non-magnetic materials to manufacture the bottom hole assembly can overcome the magnetic influence from the bottom hole assembly but are still subject to influence from the information and this can render such a system inaccurate.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome, or at least ameliorate, one or more of the deficiencies of the prior art mentioned above, or to provide the consumer with a useful or commercial choice.

Other objects and advantages of the present invention will become apparent from the following description; taken in connection with the accompanying drawings, wherein, by way of illustration and example, a preferred embodiment of the present invention is disclosed.

According to a first broad aspect of the present invention there is provided an apparatus for use in a downhole survey in conjunction with core sampling, the apparatus comprising a body adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the apparatus and operated in the borehole.

The downhole surveying instrument may comprise a downhole survey tool or a component thereof.

The downhole surveying instrument may, for example, comprise a downhole tool comprising one or more sensor devices such as orthogonal accelerometers, magnetometers, gyroscopes, MEMS (microelectromechanical) gyro sensors, or any combination thereof.

A suitable downhole surveying instrument may comprise the downhole tool as described and illustrated in International application PCT/AU2011/000628, the contents of which are incorporated herein by way of reference. Other appropriate downhole tools can, of course, also be used.

Where the downhole surveying instrument comprises a geo-magnetic device, it may be necessary for the body, or at least relevant parts thereof, to be made of material or materials which do not interfere magnetically with the geo-magnetic device. In particular, the body, or at least relevant parts thereof, may need to be made of material which is non-magnetic.

Where the downhole surveying instrument does not comprise a geo-magnetic device, the body would not necessarily need to be made of materials which are non-magnetic. The downhole surveying tool described and illustrated in International application PCT/AU2011/000628 is an example of an arrangement in which the body would not necessarily need to be made of materials which are non-magnetic.

The body may incorporate a cavity for accommodating the downhole surveying instrument.

Preferably, the body is configured to facilitate installation of the downhole surveying instrument in the cavity. The body may, for example, be constructed in two or more parts, with one part being selectively separable from another part to provide access to the cavity.

The body may be adapted to provide cushioning to afford some impact protection for the surveying instrument. In particular, the body may be configured to cushion impact forces following descent into a borehole.

The cushioning may be provided by a cushioning mechanism incorporated in the body.

The cushioning mechanism may comprise an elastic structure such as a spring for absorbing a shock impact. The cushioning mechanism may further comprise a shock absorber for damping the spring oscillations.

Magnetic braking may be utilised to slow the apparatus as it approaches the end of its descent into a borehole. The apparatus may for example include at least a portion of a magnetic braking system for slowing the apparatus in this manner.

The apparatus may have provision for controlled positioning thereof within the borehole for operation of the surveying instrument. The controlled positioning provides stable support within the borehole.

Preferably, said provision for controlled positioning within the borehole for operation of the surveying instrument comprises means for engaging the surrounding portion of a drill string to provide stabilization with respect to the drill string.

The engaging means may be adapted to support the apparatus within the surrounding portion of the drill string in a circumferentially centred manner. With such an arrangement, the engaging means may be configured as a centraliser.

The engaging means may comprise radially disposed arms biased outwardly to contact sides of the drill string, thus centrally positioning the apparatus within the surrounding portion of the drill string. The arms may each include an outer contact portion configured for contact with the inner wall of the surrounding portion of the drill string. The outer contact portion may be of any appropriate form, such as a pad or a roller.

In one arrangement, the engaging means may be configured to engage the drill string for movement therealong as the apparatus is raised or lowered within the borehole. With this arrangement, the engaging means may comprise a fixed structure configured as a carriage for movement along the internal wall of the drill string.

In another arrangement, the engaging means may be adapted for movement between collapsed and extended conditions, whereby in the collapsed condition it is clear of the internal wall of the drill string for movement therealong and in the extended condition it is in engagement with the drill string for controlled positioning of the apparatus within the borehole to provide stable support for operation of the surveying instrument.

Actuation means may be provided for actuating the engaging means. The actuation means may be triggered by contact, or at least proximity, between the apparatus and another downhole member. By way of example, the actuation of the engaging means may be triggered by contact between an inner tube assembly and an overshot assembly.

The actuation means may be of any appropriate form; for example, the actuation means may comprise a mechanical actuator or some other arrangement such as magnetic switch, proximity detector or wireless transmission system

The apparatus may also be adapted to initiate operation of the downhole surveying instrument. By way of example, the apparatus may be adapted to initiate operation of the downhole surveying instrument as it approaches or completes the end of its descent into the borehole. In this regard, the apparatus may comprise means such as a shock logger for measuring rapid deceleration and transmitting a command to the downhole surveying instrument.

The apparatus may be of any appropriate form. The apparatus may, for example, comprise an inner tube assembly of a core drill, an overshot assembly, or an arrangement such as a sub or a drop tool assembly adapted for attachment to an inner tube assembly and/or to an overshot assembly.

According to a second broad aspect of the present invention there is provided an inner tube assembly comprising a body having an upper portion adapted for connection to a retrieval system and a lower portion adapted to receive a core sample during a drilling operation, the body being adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the inner tube assembly and operated in the borehole.

Preferably, the retrieval system comprises an overshot assembly attached to the end of a wireline.

Preferably, the upper portion of the body is configured for engagement with the overshot assembly in known manner.

Preferably, the overshot assembly incorporates means operable for controlled positioning of the body within the borehole for operation of the surveying instrument. The controlled positioning provides stable support within the borehole.

More particularly, said means operable for controlled positioning of the body within the borehole comprises means for controlling positioning of the overshot assembly within the borehole. With this arrangement, it is the controlled positioning of the overshot assembly within the borehole that effectively controls the position of the body within the borehole for operation of the surveying instrument by virtue of the connection between the core tube assembly and the overshot assembly. In other words, controlling the positioning of the overshot assembly within the borehole has the effect of controlling the positioning of the core inner tube assembly, and hence the position of the body of the core inner tube assembly in which the downhole surveying instrument is accommodated.

The means operable for controlled positioning of the body within the borehole may comprise means for engaging the surrounding portion of a drill string to stabilise the overshot assembly with respect to the drill string.

The engaging means may be adapted to support the overshot assembly within the surrounding portion of the drill string in a circumferentially centred manner. With such an arrangement, the engaging means may be configured as a centraliser.

The engaging means may comprise radially disposed arms biased outwardly to contact sides of the drill string, thus centrally positioning the overshot assembly within the surrounding portion of the drill string. The arms may each include an outer contact portion configured for contact with the inner wall of the surrounding portion of the drill string. The outer contact portion may be of any appropriate form, such as a pad or a roller.

In one arrangement, the engaging means may be configured to engage the drill string for movement therealong as the overshot assembly is raised or lowered within the borehole. With this arrangement, the engaging means may comprise a fixed structure configured as a carriage for movement along the internal wall of the drill string.

In another arrangement, the engaging means may be adapted for movement between collapsed and extended conditions, whereby in the collapsed condition it is clear of the internal wall of the drill string for movement therealong and in the extended condition it is in engagement with the drill string for controlled positioning of the overshot assembly within the borehole to provide stable support for operation of the surveying instrument.

According to a third broad aspect of the present invention there is provided an overshot assembly comprising a body having an upper portion adapted for connection to a wireline and a lower portion adapted to be releasably connected to an inner tube assembly, the body being adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the overshot assembly and operated in the borehole.

The downhole surveying instrument may comprise a downhole survey tool or a component thereof.

The downhole surveying instrument may, for example, comprise a downhole tool comprising one or more sensor devices such as orthogonal accelerometers, magnetometers, gyroscopes, MEMS (microelectromechanical) gyro seniors, or any combination thereof.

A suitable downhole surveying instrument may comprise the downhole tool as described and illustrated in International application PCT/AU2011/000628, the contents of which are incorporated herein by way of reference. Other appropriate downhole tools can, of course, also be used.

Where the downhole surveying instrument comprises a geo-magnetic device, it may be necessary for the body, or at least relevant parts thereof, to be made of material or materials which do not interfere magnetically with the geo-magnetic device. In particular, the body, or at least relevant parts thereof, may need to be made of material which is non-magnetic.

Where the downhole surveying instrument does not comprise a geo-magnetic device, the body would not necessarily need to be made of materials which are non-magnetic. The downhole surveying tool described and illustrated in International application PCT/AU20111000628 is an example of an arrangement in which the body would not necessarily need to be made of materials which are non-magnetic.

Preferably, the downhole surveying instrument is operable within the borehole while the overshot assembly is connected to the inner tube assembly.

Preferably, the overshot assembly further comprises means operable for controlled positioning of the body within the borehole for operation of the surveying instrument.

The controlled positioning provides stable support within the borehole.

The means operable for controlled positioning of the body within the borehole may comprise means for engaging the surrounding drill string to stabilise the body.

The engaging means may be adapted to support the body within the drill string in a circumferentially centred manner. With such an arrangement, the engaging means may be configured as a centraliser.

The engaging means may comprise radially disposed arms biased outwardly to contact sides of the drill string, thus centrally positioning the body within the drill string. The arms may each include an outer contact portion configured for contact with the inner wall of the surrounding drill string. The outer contact portion may be of any appropriate form, such as a pad or a roller.

In one arrangement, the engaging means may be configured to engage the drill string for movement therealong as the overshot assembly is raised or lowered within the borehole. With this arrangement, the engaging means may comprise a fixed structure configured as a carriage for movement along the internal wall of the drill string.

In another arrangement, the engaging means may be adapted for movement between collapsed and extended conditions, whereby in the collapsed condition it is clear of the internal wall of the drill string for movement therealong and in the extended condition it is in engagement with the drill string for controlled positioning of the body within the borehole to provide stable support for operation of the surveying instrument.

The body may incorporate a cavity for accommodating the downhole surveying instrument.

Preferably, the body is configured to facilitate installation of the downhole surveying instrument in the cavity. The body may, for example, be constructed in two or more parts, with one part being selectively separable from another part to provide access to the cavity.

Preferably, the body is adapted to provide cushioning to afford some impact protection for the surveying instrument. In particular, the body is preferably configured to cushion impact forces when moving into contact with a downhole inner tube assembly following descent on the wireline.

The cushioning may be provided by a cushioning mechanism incorporated in the body.

The cushioning mechanism may comprise an elastic structure such as a spring for absorbing a shock impact. The cushioning mechanism may further comprise a shock absorber for damping the spring oscillations.

The cushioning mechanism may alternatively, or additionally, comprise a parachute, or other controlled descent system or method.

The cushioning mechanism may be incorporated within the body between two parts thereof.

Magnetic braking may be utilised to slow the overshot assembly as it approaches the end of its descent into a borehole. The overshot assembly may for example include at least a portion of a magnetic braking system for slowing the overshot assembly in this manner.

Preferably, the lower portion of the body incorporates a latching mechanism for releasable connection to a mating formation on the inner tube assembly. Typically, the mating formation is configured as a spear and the latching mechanism comprises latching dogs.

The body may incorporate an actuator for actuating the latching mechanism to engage/disengage the formation upon engagement between the overshot assembly and the inner core tube.

Preferably, the overshot assembly is provided with a means for allowing a tool face of the downhole surveying instrument to be transferred to an external surface of the overshot assembly. For example, an external surface of the body of the overshot assembly may include a tool face mark, and one of the body and a pressure barrel of the surveying instrument may include a locating lug which is receivable in a groove in the other of the body and the pressure barrel of the surveying instrument so that the tool face of the instrument can thereby be synchronised with the tool face mark on the external surface of the body. Where the downhole surveying instrument comprises one or more gyroscopes, the tool face of the downhole surveying instrument may comprise a gyro tool face, and the tool face mark on the external surface of the body may comprise a gyro tool face mark.

The overshot assembly may be provided with a means for allowing the tool face to be adjustably transferred to the inner tube assembly.

According to a fourth broad aspect of the present invention there is provided an apparatus adapted for connection to an inner tube assembly, the apparatus comprising a body being adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the inner tube assembly and operated in the borehole.

The apparatus according to the fourth broad aspect of the present invention may comprise a sub or a drop tool assembly attachable to the inner tube assembly.

According to a fifth broad aspect of the present invention there is provided a drop tool assembly comprising a body having an upper portion adapted to be releasably connected to a retrieval system and a lower portion adapted to be releasably connected to an inner tube assembly, the body being adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the drop tool assembly and operated in the borehole.

Preferably, the retrieval system comprises an overshot assembly attached to the end of a wireline.

Preferably, the upper portion of the body is configured for engagement with the overshot assembly in known manner.

Preferably, the drop tool assembly incorporates means operable for controlled positioning of the body within the borehole for operation of the surveying instrument. The controlled positioning provides stable support within the borehole.

The means operable for controlled positioning of the body within the borehole may comprise means for engaging the surrounding drill string to stabilise the body.

The engaging means may be adapted to support the body within the drill string in a circumferentially centred manner. With such an arrangement, the engaging means may be configured as a centraliser.

The engaging means may comprise radially disposed arms biased outwardly to contact sides of the drill string, thus centrally positioning the body within the drill string. The arms may each include an outer contact portion configured for contact with the inner wall of the surrounding drill string. The outer contact portion may be of any appropriate form, such as a pad or a roller.

In one arrangement, the engaging means may be configured to engage the drill string for movement therealong as the drop tool assembly is raised or lowered within the borehole. With this arrangement, the engaging means may comprise a fixed structure configured as a carriage for movement along the internal wall of the drill string.

In another arrangement, the engaging means may be adapted for movement between collapsed and extended conditions, whereby in the collapsed condition it is clear of the internal wall of the drill string for movement therealong and in the extended condition it is in engagement with the drill string for controlled positioning of the body within the borehole to provide stable support for operation of the surveying instrument.

The body may incorporate a cavity for accommodating the downhole surveying instrument.

Preferably, the body is configured to facilitate installation of the downhole surveying instrument in the cavity. The body may, for example, be constructed in two or more parts, with one part being selectively separable from another part to provide access to the cavity.

Magnetic braking may be utilised to slow the drop tool assembly as it approaches the end of its descent into a borehole. The drop tool assembly may for example include at least a portion of a magnetic braking system for slowing the drop tool assembly in this manner.

Preferably, the body has an upper portion adapted to be releasably connected to an overshot assembly, and a lower portion adapted to be releasably connected to an inner tube assembly.

Preferably, the lower portion of the body incorporates a latching mechanism for releasable connection to a mating formation on the inner tube assembly. Typically, the mating formation is configured as a spear and the latching mechanism comprises latching dogs.

The body may incorporate an actuator for actuating the latching mechanism to engage the formation upon engagement between the drop tool assembly and the inner core tube.

Preferably, the upper portion of the body incorporates a mating formation. It is preferred that the mating formation is configured as a spearhead point.

The downhole surveying instrument may comprise a downhole survey tool or a component thereof.

The downhole surveying instrument may, for example, comprise a downhole tool comprising one or more sensor devices such as orthogonal accelerometers, magnetometers, gyroscopes, MEMS (microelectromechanical) gyro sensors, or any combination thereof.

A suitable downhole surveying instrument may comprise the downhole tool as described and illustrated in International application PCT/AU2011/000628, the contents of which, as mentioned above, are incorporated herein by way of reference. Other appropriate downhole tools can, of course, also be used.

Where the downhole surveying instrument comprises a geo-magnetic device; it may be necessary for the body, or at least relevant parts thereof, to be made of material or materials which do not interfere magnetically with the geo-magnetic device. In particular, the body or at least relevant parts thereof, may need to be made of material which is non-magnetic.

Where the downhole surveying instrument does not comprise a geo-magnetic device, the body would not necessarily need to be made of materials which are non-magnetic. The downhole surveying tool described and illustrated in International application PCT/AU2011/000628 is an example of an arrangement in which the body would not necessarily need to be made of materials which are non-magnetic.

The downhole surveying instrument may be operable within the borehole either while the drop tool assembly is connected to the inner tube assembly, or when the drop tool assembly is not connected to the inner tube assembly.

Preferably, the drop tool assembly is provided with a means for allowing a tool face of the downhole surveying instrument to be effectively transferred to an external surface of the drop tool assembly. For example, an external surface of the body of the drop tool assembly may include a tool face mark, and one of the body and a pressure barrel of the surveying instrument may include a locating lug which is receivable in a groove in the other of the body and the pressure barrel of the surveying instrument so, that the tool face of the instrument can thereby be synchronised with the tool face mark on the external surface of the body. Where the downhole surveying instrument comprises one or more gyroscopes, the tool face of the downhole surveying instrument may comprise a gyro tool face, and the tool face mark on the external surface of the body may comprise a gyro tool face mark.

The drop tool assembly may be provided with a means for allowing the tool face to be adjustably transferred to the inner tube assembly.

Preferably, the drop tool assembly also comprises a water pressure activation system for indicating when the drop tool assembly has landed in or otherwise reached its final position within a borehole. It is preferred that the water pressure activation system is operable to activate/deactivate the downhole surveying instrument.

According to a sixth broad aspect of the present invention there is provided a downhole assembly comprising an inner tube assembly according to the second broad aspect of the present invention in combination with an overshot assembly.

Preferably, the downhole assembly further comprises a release system for allowing the overshot assembly to be released from the inner tube assembly while the overshot assembly and the inner tube assembly are located downhole.

Preferably, the downhole assembly also comprises a synchronisation system for allowing the overshot assembly to be connected to the inner tube assembly so that a predetermined tool face of the overshot assembly is synchronised to the inner tube assembly. It is preferred that the predetermined tool face is a predetermined gyro tool face of the overshot assembly.

Preferably, the synchronisation system comprises a profiled portion of the overshot assembly and a profiled portion of the inner tube assembly, the profiled portions being configured to engage with each other such that the overshot assembly is able to rotate relative to the inner tube assembly into a home position under its own weight or with minimal thrust. It is preferred that each profiled portion comprises a respective mule shoe.

Preferably, the downhole assembly further comprises a locking system for mechanically locking the inner tube assembly to inturn synchronise to a tool face of the overshot assembly. It is preferred that the locking system comprises a flow clutch.

Alternatively, in another arrangement, a tool face position of the inner tube assembly is able to be synchronised to the overshot assembly by wireless transmission.

Preferably the overshot assembly in the combination according to the sixth broad aspect of the present invention comprises means operable for controlled positioning of the body within the borehole for operation of the surveying instrument.

In this regard, the overshot assembly may incorporate any one or more of the features described above.

According to a seventh broad aspect of the present invention there is provided a downhole assembly comprising a drop tool assembly according to the fifth broad aspect of the present invention in combination with at least one of an inner tube assembly and an overshot assembly.

Preferably, the downhole assembly comprises a release system for allowing the drop tool assembly to be released from the inner tube assembly while the drop tool assembly is located down hole.

Preferably, the downhole assembly also comprises a synchronisation system for allowing the drop tool assembly to be connected to the inner tube assembly so that a predetermined tool face of the drop tool assembly is synchronised to the inner tube assembly. It is preferred that the predetermined tool face is a predetermined gyro tool face of the drop tool assembly.

Preferably, the synchronisation system comprises a profiled portion of the drop tool assembly and a profiled portion of the inner tube assembly, the profiled portions being configured to engage with each other such that the drop tool assembly is able to rotate relative to the inner tube assembly into a home position under its own weight or with minimal thrust. It is preferred that each profiled portion comprises a respective mule shoe.

Preferably, the downhole assembly further comprises a locking system for mechanically locking the inner tube assembly to inturn synchronise to a tool face of the drop tool assembly. It is preferred that the locking system comprises a flow clutch.

Alternatively, in another arrangement, a tool face position of the inner tube assembly is able to be synchronised to the drop tool assembly by wireless transmission.

According to an eighth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using a surveying instrument accommodated in an apparatus according to the first broad aspect of the present invention.

According to a ninth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using a surveying instrument accommodated in an inner tube assembly according to the second broad aspect of the present invention.

According to tenth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using a surveying instrument accommodated in an overshot assembly according to the third broad aspect of the present invention.

According to an eleventh broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using a surveying instrument accommodated in an apparatus according to the fourth broad aspect of the present invention.

According to a twelfth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using a surveying instrument accommodated in a drop tool assembly according to the fifth broad aspect of the present invention.

According to thirteenth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using a surveying instrument accommodated in an inner tube assembly forming part of a combination according to the sixth broad aspect of the present invention.

According to a fourteenth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using a surveying instrument accommodated in a drop tool assembly forming part of a combination according to the seventh broad aspect of the present invention.

According to a fifteenth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using apparatus incorporating an onboard downhole, surveying instrument, the method comprising lowering the apparatus down the borehole, taking a measurement down the borehole using the onboard downhole surveying instrument, and retrieving a core sample using the apparatus.

The method may further comprise actuating the onboard downhole surveying instrument to take the measurement in response to the apparatus approaching or completing the end of its descent into the borehole.

Typically, the apparatus comprises an overshot assembly.

According to sixteenth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using an inner tube assembly incorporating an onboard downhole surveying instrument, the method comprising deploying the inner tube assembly in the borehole, taking a measurement down the borehole using the onboard downhole surveying instrument, and retrieving the inner tube assembly containing a core sample.

Preferably, the core inner tube assembly incorporating the onboard downhole surveying instrument is retrieved from the borehole using an overshot assembly.

Preferably, the method further comprises controlling the position of the core inner tube assembly within the borehole for operation of the surveying instrument.

Preferably, the act of controlling the position of the core inner tube assembly within the borehole for operation of the surveying instrument comprises controlling the positioning of the overshot assembly within the borehole.

According to a seventeenth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using an overshot assembly according to the third broad aspect of the present invention.

According to an eighteenth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using an overshot assembly incorporating an onboard downhole surveying instrument, the method comprising lowering the overshot assembly down the borehole, taking a measurement down the borehole using the onboard downhole surveying instrument, and retrieving an inner tube assembly containing a core sample using the overshot assembly.

According to a nineteenth broad aspect of the present invention there is provided a method of conducting a borehole surveying operation using a drop tool assembly incorporating an onboard downhole surveying instrument, the method comprising lowering the drop tool assembly down the borehole using the onboard downhole surveying instrument, and retrieving an inner tube assembly containing a core sample using the drop tool assembly and an overshot assembly.

The term “measurement” as used herein is taken to include any reading, data, signal, or other input or collection of inputs received by the downhole surveying instrument.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings in which:

FIG. 1 is a schematic side view of an inner tube assembly;

FIG. 2 is a schematic side view of an overshot assembly for use in combination with the inner tube assembly;

FIG. 3 is a fragmentary side view of the bottom end of a drill string incorporating a drill barrel connected to a series of drill rods, with the inner tube assembly being receivable in the drill barrel in known manner;

FIG. 4 is a schematic side view of another overshot assembly;

FIG. 5 depicts an overshot assembly and an inner tube assembly of a downhole assembly;

FIG. 6 depicts a drop tool assembly and an inner tube assembly of a downhole assembly;

FIG. 7 depicts a portion of the drop tool assembly body, and a portion of the surveying instrument pressure barrel; and

FIG. 8 depicts a water pressure activation system of the drop tool assembly.

DESCRIPTION OF EMBODIMENTS

In the drawings, like features have been referenced with like reference numbers.

The preferred embodiment shown in FIGS. 1 to 3 of the drawings is directed to an inner tube assembly 10 for core retrieval in core drilling operations for borehole surveys. The inner tube assembly 10 is configured to accommodate an onboard downhole surveying instrument, as will be explained in more detail later. This arrangement allows for coordination between downhole surveying and core retrieval operations in order to reduce downtime while the procedures are being performed,

The core drilling operation is performed with a core drill comprising an outer tube assembly 13 in conjunction with the inner tube assembly 10. The outer tube assembly 13 is referred to as the drilling barrel and is fitted as a bottom end assembly 15 to a series of drill rods or drill pipes 17 which constitute a drill string.

The inner tube assembly 10 comprises a body 21 having a bottom end section 23 and a top end section 25. The bottom end section 23 incorporates a core inner tube (not shown) to progressively receive a core sample during a core drilling operation in known manner.

The top end section 25 incorporates a backend assembly which includes a spearhead point 27 releasably engagable with an overshot assembly 30 in known manner. With this arrangement, the inner tube assembly 10 can be lowered into, and retrieved from, the outer tube assembly 13 and the associated drill string 17.

The body 21 defines an internal compartment 33 adapted to accommodate a downhole survey instrument 35, as shown in FIG. 1. In this way, the downhole survey instrument 35 is onboard the inner tube assembly 10. The downhole survey instrument 35 would typically comprise a downhole tool having one or more sensor devices such as orthogonal accelerometers, magnetometers, gyroscopes, MEMS (microelectromechanical) gyro sensors, or any combination thereof. In this preferred embodiment, the compartment 33 is configured to receive a downhole tool of the type described and illustrated in international application PCT/AU2011/000628, the contents of which are incorporated herein by way of reference.

The body 21 may incorporate means (not shown) such as an IR port or other telemetry arrangement for communication with the downhole survey instrument to retrieve measurements therefrom.

The body 21 comprises an upper portion 41 and a lower portion 43 adapted to be releasably connected together by a fluid-tight connection 45. The connection 45 may comprise a threaded connection and an associated fluid seal arrangement. The two portions 41, 43 cooperate to define the internal compartment 33 adapted to receive and accommodate the downhole survey instrument 35. The two portions 41, 43 can be separated by disengaging the connection 45 to provide access to the compartment 33 for insertion and removal of the downhole survey instrument 35.

The core inner tube assembly 10 can be used with a conventional overshot. Assembly.

However, it may be advantageous to use the core inner tube assembly 10 in conjunction with an overshot assembly which has a facility to provide controlled positioning of the body 21 of the inner tube assembly 10 within the borehole for operation of the onboard surveying instrument 35. The controlled positioning is intended to provide stable support of the body 21 within the drill string 17 in order to allow the onboard surveying instrument 35 to take measurements which are not affected by movement and vibration.

The overshot assembly 30 shown in FIG. 2 is provided with such a facility to provide controlled positioning of the body 21 within the borehole.

In the arrangement illustrated in FIG. 2, the overshot assembly 30 comprises a body 51 having a lower end section 53 and an upper end section 55.

The lower end section 53 of the body 51 incorporates a latching mechanism 57 for releasable connection to the spearhead point 27 on the inner tube assembly 10. The latching mechanism 57 may comprise latching dogs of known kind for releasable engagement with the spearhead point 27. The body may also incorporate an actuator (not shown) for actuating the latching mechanism 57 to engage/disengage the spearhead point 27 upon engagement between the overshot assembly 30 and the inner tube assembly 10.

The upper end section 55 of the body 51 is, configured for attachment to a wireline cable 59 of known kind, as shown in FIG. 2.

The overshot assembly 30 further comprises means 60 operable for the above-mentioned controlled positioning of the body 21 of the inner tube assembly 10 within the borehole. More particularly, such means 60 is operable for controlling positioning of the overshot assembly 30 within the borehole. With this arrangement, it is the controlled positioning of the overshot assembly 30 within the borehole that effectively controls the position of the body 21 of the inner tube assembly 10 within the borehole by virtue of the connection between the core tube assembly 10 and the overshot assembly 30. In other words, controlling the positioning of the overshot assembly 30 within the borehole has the effect of controlling the positioning of the core tube assembly 10, and hence the position of the body 21 of the core tube assembly 10 in which the downhole surveying instrument 35 is accommodated.

The position of the overshot assembly 30 is controlled by controlling the position of the overshot body 51 within the drill string 17. For this purpose, the means 60 comprises engaging means 61 provided on the overshot assembly 30 for engaging the surrounding section of the drill string 17 to stabilise the overshot body 51.

The engaging means 61 is configured as a centraliser 63 which is adapted to support the body 51 within the drill string 17 in a circumferentially centred manner.

The centraliser 63 comprises a series of radially disposed arms 65 adapted to extend outwardly to contact sides of the drill rods 17, thus centrally positioning the body 51 within the surrounding section of the drill string. The radially disposed arms 65 are circumferentially spaced. In this preferred embodiment, there are four radially disposed arms although other arrangements may be used.

The arms 65 each include an outer contact portion 67 configured for contact with the inner wall of the surrounding section of the drill string. The outer contact portion 67 may be of any appropriate form, such as a pad or a roller.

In the arrangement shown, the arms 65 are adapted for movement between collapsed and extended conditions, whereby in the collapsed condition each arm is clear of the internal wall for the drill rods and in the extended condition it is in engagement with the drill rods for controlled positioning of the body 51 within the borehole to provide stable support for operation of the surveying instrument 35 onboard the core inner tube assembly 10.

In operation, the core drill operates in the normal way. A core is generated during the drilling operation, with the core progressively extending along the core inner tube within the inner tube assembly 10 as drilling progresses. When a core sample is required, the core within the core inner tube is fractured to provide the core sample. The inner tube assembly 10 and the fractured core sample contained therein are then retrieved from within the drill hole using the overshot assembly 30 which is lowered down to the inner tube assembly on the wireline cable 59. As the overshot assembly 30 contacts the inner tube assembly 10, the latching mechanism 57 engages the spearhead point 27 on the inner tube assembly.

Once the overshot assembly 30 has been connected to the inner tube assembly 10, the centraliser 63 can be actuated as desired to support the body 51 within the surrounding section of the drill string 17 in a circumferentially centred manner. This controlled positioning provides stable support for the body 21 of the inner tube assembly 10 within the drill string in order to allow the onboard surveying instrument 35 to take measurements which are not affected by movement and vibration. Once the measurements are taken, the centraliser 63 can be released and the overshot assembly 30 can be raised using the wireline cable 59 to complete the core retrieval process in the conventional manner.

The surveying instrument 35 onboard the inner tube assembly 10 can be interrogated as required to retrieve recorded data.

It is a particular feature of the preferred embodiment that the surveying instrument 35 is onboard the inner tube assembly 10, thereby allowing surveying measurements to be taken during the core collection and retrieval process as desired. In other words, the taking of surveying measurements can be integrated with the core collection and retrieval process, rather than being a separate operation as is conventional practice. This is advantageous, as it can reduce the downtime during which drilling operations need to be suspended in order for core samples to be retrieved and surveying measurements to be taken.

In this preferred embodiment, the downhole instrument 35 comprising the tool of the type described and illustrated in international application PCT/AU2011/000628 is not affected by magnetic materials in the presence of the environment of its use, and so the body can be constructed of any appropriate material; that is, it is not necessary to use non-magnetic materials such as CuBe in the construction of the body 21 of the inner tube assembly 10.

In other preferred embodiments with which a geomagnetic device is used as the survey instrument, it may be necessary for the body 21, or at least relevant parts thereof, to be made of material or materials which do not interfere magnetically with the geo-magnetic device. In particular, the body, or at least relevant parts thereof, may need to be to be made of material which is non-magnetic.

The preferred embodiment shown in FIG. 4 of the drawings is directed to an overshot assembly 100 for use in a core drilling operation in a borehole survey.

The core drilling operation is performed with a core drill (not shown) fitted as a bottom end assembly to a series of drill rods. The core drill comprises an inner tube assembly, which includes a core tube, for core retrieval. The core drill also comprises an outer tube assembly.

The inner tube assembly further comprises a backend assembly which includes a spearhead point releasably engagable with the overshot assembly 100. In FIG. 4, the spearhead point is depicted schematically and identified by reference numeral 101.

With this arrangement, the inner tube assembly can be lowered into, and retrieved from, the outer tube assembly and the drill string to which the outer tube assembly is incorporated.

The overshot assembly 100 comprises a body 105 having a lower end 107 and an upper end 109. The body defines an internal compartment 111 adapted to accommodate a downhole survey instrument. In this way, the downhole survey instrument is onboard the overshot assembly 100. The downhole survey instrument would typically comprise a downhole tool having one or more sensor devices such as orthogonal accelerometers, magnetometers, gyroscopes, MEMS (microelectromechanical) gyro sensors, or any combination thereof. In this preferred embodiment, the compartment 111 is configured to receive a downhole tool of the type described and illustrated in international application PCT/AU2011/000628, the contents of which are incorporated herein by way of reference.

The body 105 incorporates means 108 such as an IR port or other telemetry method for communication with the downhole survey instrument to retrieve measurements therefrom.

The body 105 comprises an upper portion 113 and a lower portion 115, with the two portions being interconnected by a cushioning mechanism 117 adapted to provide cushioning to afford some impact protection for the downhole survey instrument accommodated in the compartment 111. In particular, the cushioning mechanism 117 is adapted to cushion impact forces when the overshot assembly 100 descends into contact with the inner tube assembly.

In the arrangement illustrated in FIG. 4, the cushioning mechanism 117 comprises an elastic structure 119 configured as a spring for absorbing a shock impact. The cushioning mechanism 117 further comprises a shock absorber 121 for damping the spring oscillations. The shock absorber 121 may be of any appropriate type, such as an arrangement adapted for controlled displacement of damping fluid (comprising, for example, air and oil) to effect a damping action.

The compartment 111 is incorporated in the upper portion 113. The upper portion 113 comprises two sections, being a top section 123 and a bottom section 125 adapted to be releasably connected together by fluid-tight connection 127. The connection 127 may comprise a threaded connection and an associated fluid seal arrangement. The two sections 123, 125 cooperate to define the compartment 111 adapted to receive and accommodate the downhole survey instrument. The two sections 123, 125 are selectively separable to provide access to the compartment 111.

The lower end 107 of the body 105 incorporates a latching mechanism 131 for releasable connection to the spearhead point 101 on the inner tube assembly. In this embodiment, the latching mechanism 131 comprises latching dogs of known kind for releasable engagement with the spearhead point 101.

The body 105 also incorporates an actuator (not shown) for actuating the latching mechanism 131 to engage/disengage the spearhead point 101 upon engagement between the overshot assembly 100 and the inner tube assembly.

The upper end 109 of the body 105 is configured for attachment to a wireline cable of known kind. In the arrangement illustrated, the upper end 109 of the body 105 incorporates an eyelet 133 to which the wireline cable can be attached. In FIG. 4, the wireline cable is depicted schematically and identified by reference numeral 135.

The overshot assembly 100 further comprises means 140 operable for controlled positioning of the body 105 within the borehole for operation of the surveying instrument accommodated within the compartment 111. The controlled positioning is intended to provide stable support of the body 105 within the drill string in order to allow the onboard surveying instrument to take measurements which are not affected by movement and vibration.

The means 140 operable for controlled positioning of the body 105 within the borehole comprise engaging means 141 for engaging the adjacent portion of the drill string to stabilise the body.

The engaging means 141 is configured as a centraliser 143 which is adapted to support the body 105 within the drill pipe in a circumferentially centred manner.

The centraliser 143 comprises a series of radially disposed arms 145 adapted to extend outwardly to contact sides of the drill pipe, thus centrally positioning the body within the drill pipe. The radially disposed arms 145 are circumferentially spaced. In this preferred embodiment, there are four radially disposed arms 145, although other arrangements may be used.

The arms 145 each include an outer contact portion 147 configured for contact with the inner wall of the surrounding drill pipe. The outer contact portion 147 may be of any appropriate form, such as a pad or a roller.

In this preferred embodiment, the arms 145 are adapted for movement between collapsed and extended conditions, whereby in the collapsed condition each arm is clear of the internal wall for the drill pipe and in the extended condition it is in engagement with the drill pipe for controlled positioning of the body 105 within the borehole to provide stable support for operation of the surveying instrument.

In operation, the core drill operates in the normal way. A core is generated during the drilling operation, with the core progressively extending along the core inner tube within the inner tube assembly as drilling progresses. When a core sample is required, the core within the core inner tube is fractured. The inner tube assembly and the fractured core sample contained therein are then retrieved from within the drill hole using the overshot assembly 100 which is lowered down to the inner tube assembly on the wireline cable 135. As the overshot assembly 100 contacts the inner tube assembly, the latching mechanism 131 engages the spearhead point 101 on the inner tube assembly. The impact force generated upon the overshot assembly descending into contact with the inner tube assembly is cushioned by the cushioning mechanism 117 as previously explained. Once the overshot assembly 100 has been connected to the inner tube assembly, the centralises 143 is actuated to support the body 105 within the drill pipe in a circumferentially centred manner. This controlled positioning effects stable support of the body 105 within the drill pipe in order to allow the onboard surveying instrument to take measurements which are not affected by movement and vibration. Once the measurements are taken, the overshot assembly 100 can be raised using the wireline cable 135 to complete the core retrieval process in the conventional manner.

It is a particular feature of the preferred embodiment that the surveying instrument is onboard the overshot assembly 100, thereby allowing surveying measurements to be taken during the core retrieval process if desired. In other words, the taking of surveying measurements can be integrated with the core retrieval process, rather than being a separate operation as is conventional practice. This is advantageous, as it can reduce the downtime during which drilling operations need to be suspended in order for core samples to be retrieved and surveying measurements to be taken.

In this preferred embodiment, the downhole tool of the type described and illustrated in international application PCT/AU2011/000628 is not affected by magnetic materials in the presence of the environment of its use and so the body can be constructed of any appropriate material; that is, it is not necessary to use non magnetic materials such as CuBe in the construction of the body 105.

In other preferred embodiments with which a geo-magnetic device is used as the survey instrument, it may be necessary for the body, or at least relevant parts thereof, to be made of material or materials which do not interfere magnetically with the geo-magnetic device. In particular, the body, or at least relevant parts thereof, may need to be to be made of material which is non-magnetic.

Referring to FIG. 5 there is depicted an overshot assembly 200 and an inner tube assembly 201 of a downhole assembly 202.

The overshot assembly 200 can be used in a core drilling operation in a borehole survey.

The core drilling operation is performed with a core drill (not shown) fitted as a bottom end assembly to a series of drill rods or pipes called a drill string. The core drill comprises the inner tube assembly 201, which includes a core tube, for core retrieval. The core drill also comprises an outer tube assembly.

The inner tube assembly 201 further comprises a backend assembly which includes a spearhead point 101 releasably engagable with the overshot assembly 200.

With this arrangement, the inner tube assembly 201 can be lowered into, and retrieved from an outer tube assembly and the drill string to which the outer tube assembly is incorporated.

The overshot assembly 200 comprises a body 105 having a lower end 107 and an upper end 109. The body 105 defines an internal compartment/cavity 111 adapted to accommodate a downhole survey instrument. In this way, the downhole survey instrument would typically comprise a downhole tool having one or more sensor devices such as orthogonal accelerometers, magnetometers, gyroscopes, MEMS (microelectromechanical) gyro sensors, or any combination thereof. In this preferred embodiment, the compartment 111 is configured to receive a downhole tool of the type described and illustrated in International application PCT/AU2011/000628 whose contents have been incorporated herein by reference.

Although not depicted in the drawings, the body 105 may incorporate means such as an infrared (IR) port or other telemetry method for communication with the downhole survey instrument to retrieve measurements therefrom.

The body 105 comprises an upper portion 113 and a lower portion 115. Although not shown in the drawings, the two portions 113, 115 may be interconnected by a cushioning mechanism adapted to provide cushioning to afford some impact protection for the downhole survey instrument accommodated in the compartment 111. In particular, the cushioning mechanism is adapted to cushion impact forces when the overshot assembly 200 descends into contact with the inner tube assembly 201.

The cushioning mechanism could, for example, comprise an elastic structure configured as a spring for absorbing a shock impact. The cushioning mechanism may further comprise a shock absorber for damping the spring oscillations. The shock absorber may be of any appropriate type, such as an arrangement adapted for controlled displacement of damping fluid (comprising, for example, air and oil) to effect a damping action.

The compartment 111 is incorporated in the upper portion 113. The upper portion 113 comprises two sections, being a top section 123 and a bottom section 125 adapted to be releasably connected together by a fluid-tight connection 127. The connection 127 may comprise a threaded connection and an associated fluid seal arrangement. The two sections 123, 125 cooperate to define the compartment 111 adapted to receive and accommodate the downhole survey instrument. The two sections 123, 125 are selectively separable to provide access to the compartment 111.

The lower end 107 of the body 105 incorporates a latching mechanism 131 for releasable connection to the spearhead point 101 on the inner tube assembly 201. In this preferred embodiment, the latching mechanism 131 comprises latching/lifting dogs 210 of known kind for releasable engagement with the spearhead point 101.

The body 105 may also incorporate an actuator (not shown) for actuating the latching mechanism 131 to engage/disengage the spearhead point 101 upon engagement between the overshot assembly 200 and the inner tube assembly 201.

The upper end 109 of the body 105 is configured for attachment to a wireline cable 135 of known kind. In the arrangement illustrated, the upper end 109 of the body 105 incorporates an eyelet 133 to which the wireline cable 135 can be attached.

The overshot assembly 200 further comprises means 140 operable for controlled positioning of the body 105 within the borehole for operation of the surveying instrument accommodated within the compartment 111. The controlled positioning is intended to provide stable support of the body 105 within the drill string in order to allow the onboard surveying instrument to take measurements which are not affected by movement and vibration.

The means 140 operable for controlled positioning of the body 105 within the borehole comprises engaging means 141 for engaging the adjacent portion of the drill string to stabilise the body 105.

The engaging means 141 is configured as a centraliser 143 which is adapted to support the body 105 within the drill pipe in a circumferentially centred manner.

The centraliser 143 comprises a series of radially disposed arms 145 adapted to extend outwardly to contact sides of the drill pipe, thus centrally positioning the body 105 within the drill pipe. The radially disposed arms 145 are circumferentially spaced. In this preferred embodiment, there are four radially disposed arms 145, although other arrangements may be used.

The arms 145 each include an outer contact portion 147 configured for contact with the inner wall of the surrounding drill pipe. The outer contact portion 147 may be of any appropriate form, such as a pad or a roller.

The arms 145 may be adapted for movement between collapsed and extended conditions, whereby in the collapsed condition each arm 145 is clear of the internal wall of the drill pipe and in the extended condition it is in engagement with the drill pipe for controlled positioning of the body 105 within the borehole to provide stable support for operation of the surveying instrument.

Magnetic braking may be utilised to slow the overshot assembly 200 as it approaches the end of its descent into a borehole. The overshot assembly 200 may for example include at least a portion of a magnetic braking system (not depicted) for slowing the overshot assembly 200 in this manner.

The overshot assembly 200 is provided with a means 220 for allowing a tool face of the downhole surveying instrument to be transferred to an external surface of the overshot assembly 200. For example, the means 220 may include a tool face mark 221 on an external surface of the body 105, and one of the body 105 and a pressure barrel of the surveying instrument may include a locating lug (not depicted) which is receivable in a groove (not depicted) in the other of the body 105 and the pressure barrel of the surveying instrument when the surveying instrument is inserted into the compartment 111 so that the tool face of the instrument can thereby be synchronised with the tool face mark 221 on the external surface of the body 105. Where the downhole surveying instrument comprises one or more gyroscopes, the tool face of the downhole surveying instrument may comprise a gyro tool face, and the tool face mark 221 on the external surface of the body 105 may comprise a gyro tool face mark.

The overshot assembly 200 may also be provided with a means 225 for allowing the tool face to be adjustably transferred to the inner tube assembly 201. For example, the means 225 may comprise a shaft (not depicted) which connects the upper portion 113 to the lower portion 115 such that the upper and lower portions 113, 115 are able to be rotated relative to each other. Rotation of the lower portion 115 relative to the upper portion 115 allows the position of a tool face mark 226 on the lower portion 115 to be adjusted relative to the tool face mark 221 and a tool face mark 227 on the inner tube assembly 201. Once the position of the tool face mark 226 has been adjusted, two nuts 228 on the shaft can be tightened to mechanically lock the portions 113, 115 together so as to prevent relative rotation between the portions 113, 115. Where the tool face mark 221 on the external surface of the body 105 comprises a gyro tool face mark, the tool face marks 226, 227 also comprise gyro tool face marks.

In the case where the surveying instrument includes one or more gyros which provide the instrument with the ability to reference true North in any aspect/environment, this will provide the ability to orientate core or the bottom hole assembly at vertical inclinations that previously could not be done with accelerometers or magnetic sensors. By being able to detect the Earth's true North direction, this can be reference directly to an external mark (i.e. gyro tool face) on the instrument/tool to give the position of the gyro tool face in relation to true North.

The downhole assembly 202 further comprises a release system 230 for allowing the overshot assembly 200 to be released from the inner tube assembly 201 while the overshot assembly 200 and the inner tube assembly 201 are located downhole. The release system 230 is operable to disengage the dogs 210 of the latching mechanism 131 from the spearhead point 101.

Additionally, the downhole assembly 202 comprises a synchronisation system 235 for allowing the overshot assembly 200 to be connected to the inner tube assembly 201 so that a predetermined tool face 221 of the overshot assembly 200 is synchronised to the inner tube assembly 201.

The synchronisation system 235 comprises a profiled portion 236 of the lower body portion 115 of the overshot assembly 200, and a profiled portion 237 of the inner tube assembly 201. The profiled portion 236 comprises a mule shoe 238, and the profiled portion 237 comprises a mule shoe 239. The profiled portions 236, 237 are configured to engage with each other such that the overshot assembly 200 is able to rotate relative to the inner tube assembly 201 into a home position under its own weight or with only minimal thrust.

Downhole assembly 202 also comprises a locking system 245 for mechanically locking the inner tube assembly 201 to inturn synchronise to the tool face 221 of the overshot assembly 200. The locking system 245 may comprise a flow clutch which is provided as part of the inner tube assembly 201 and which is able to engage and disengage the connection between two co-linear components of a rock drilling device.

Rather than including the mechanical locking system 245, the tool face position of the inner tube assembly 201 may be synchronised to the overshot assembly 200 by wireless transmission.

In operation, the core drill operates in the normal way. A core is generated during the drilling operation, with the core progressively extending along the core inner tube within the Inner tube assembly 201 as drilling progresses. When a core sample is required, the core within the core inner tube is fractured. The inner tube assembly 201 and the fractured core sample contained therein are then retrieved from within the drill hole using the overshot assembly 200 which is lowered down to the inner tube assembly 201 on the wireline cable 135. As the overshot assembly 200 contacts the inner tube assembly 201, the latching mechanism 131 engages the spearhead point 101 on the inner tube assembly 201. The impact force generated upon the overshot assembly 200 descending into contact with the inner tube assembly 201 is cushioned by the cushioning mechanism (if present). Also, if a magnetic braking means is present, the speed of descent of the overshot assembly 200 is slowed as it approaches the inner tube assembly 201. Once the overshot assembly 200 has been connected to the inner tube assembly 201, the centraliser 143 is actuated to support the body 105 within the drill pipe in order to allow the onboard surveying instrument to take measurements which are not affected by movement and vibration. Once the measurements are taken, the overshot assembly 100 can be raised using the wireline cable 135 to complete the core retrieval process in the conventional manner.

It is a particular feature of the preferred embodiment depicted in FIG. 5 that the surveying instrument is onboard the overshot assembly 200, thereby allowing surveying measurements to be taken during the core retrieval process if desired. In other words, the taking of surveying measurements can be integrated with the core retrieval process, rather than being a separate operation as is conventional practice. This is advantageous, as it can reduce the downtime during which drilling operations need to be suspended in order for core samples to be retrieved and surveying measurements to be taken.

In this preferred embodiment, the downhole tool of the type described and illustrated in International application PCT/AU2011/000628 is not affected by magnetic materials in the presence of the environment of its use and so the body can be constructed of any appropriate material; that is, it is not necessary to use non-magnetic materials such as Cube in the construction of the body 105.

In other embodiments with which a geo-magnetic device is used as the survey instrument, it may be necessary for the body 105, or at least relevant parts thereof, to be made of material or materials which do not interfere magnetically with the geo-magnetic device. In particular, the body, or at least relevant parts thereof, may need to be made of material which is non-magnetic.

Referring to FIG. 6 there is depicted a drop tool assembly 250 and an inner tube assembly 201 of a downhole assembly 252.

The drop tool assembly 250 may be used in a core drilling operation in a borehole survey in which case the drop tool assembly 250 may be releasably attached to the inner tube assembly 201.

The drop tool assembly 250 comprises a body/barrel 255 having a lower end 257 and an upper end 259. The body 255 defines an internal compartment/cavity 261 adapted to accommodate a downhole survey instrument. In this way, the downhole survey instrument would typically comprise a downhole tool having one or more sensor devices such as orthogonal accelerometers, magnetometers, gyroscopes, MEMS (microelectromechanical) gyro sensors, or any combination thereof. In this preferred embodiment, the compartment 261 is configured to receive a downhole tool of the type described and illustrated in International application PCT/AU2011/000628 whose contents have been incorporated herein by reference.

The body 255 incorporates means such as an infrared (IR) port 262 or other telemetry method for communication with the downhole survey instrument to retrieve measurements therefrom.

The body 255 comprises an upper portion 263 and a lower portion 265. Although not shown in the drawings, the two portions 263, 265 may be interconnected by a cushioning mechanism adapted to provide cushioning to afford some impact protection for the downhole survey instrument accommodated in the compartment 261. In particular, the cushioning mechanism is adapted to cushion impact forces when the drop tool assembly 250 descends into contact with the inner tube assembly 201.

The cushioning mechanism could, for example, comprise an elastic structure configured as a spring for absorbing a shock impact. The cushioning mechanism may further comprise a shock absorber for damping the spring oscillations. The shock absorber may be of any appropriate type, such as an arrangement adapted for controlled displacement of damping fluid (comprising for example, air and oil) to effect damping action.

The compartment 261 is incorporated in the upper portion 263. The upper portion 263 comprises two sections, being a top section 273 and a bottom section 275 adapted to be releasably connected together by a fluid-tight connection 277. The connection 277 may comprise a threaded connection and an associated fluid seal arrangement. The two sections 273, 275 are selectively separable to provide access to the compartment 261.

The lower end 257 of the body 255 incorporates a latching mechanism 281 for releasable connection to the spearhead point 101 on the inner tube assembly 201. In this preferred embodiment, the latching mechanism 281 comprises latching/lifting dogs 282 of known kind for releasable engagement with the spearhead point 101.

The body 255 may also incorporate an actuator (not shown) for actuating the latching mechanism 281 to engage/disengage the spearhead point 101 upon engagement between the drop tool assembly 250 and the inner tube assembly 201.

The upper end 259 of the body 255 incorporates a mating formation 283 which is configured as a spearhead point 284 which is releasably engagable with an overshot assembly (not depicted) which may also be part of the downhole assembly 251. The overshot assembly could be of any known type, it could even be one of the previously described overshot assemblies.

The drop tool assembly 250 further comprises means 290 operable for controlled positioning of the body 255 within the borehole for operation of the surveying instrument accommodated within the compartment 261. The controlled positioning is intended to provide stable support of the body 255 within the drill string in order to allow the onboard surveying instrument to take measurements which are not affected by movement and vibration.

The means 290 operable for controlled positioning of the body 255 within the borehole comprises engaging means 291 for engaging the adjacent portion of the drill string to stabilise the body 255.

The engaging means 291 is configured as a centraliser 293 which is adapted to support the body 255 within the drill pipe in a circumferentially centred manner.

The centraliser 293 comprises a series of radially disposed arms 295 adapted to extend outwardly to contact sides of the drill pipe, thus centrally positioning the body 255 within the drill pipe. The radially disposed arms 295 are circumferentially spaced. In this preferred embodiment, there are four radially disposed arms 295, although other arrangements may be used.

The arms 295 each include an outer contact portion 297 configured for contact with the inner wall of the surrounding drill pipe. The outer contact portion 297 may be of any appropriate form, such as a pad or a roller.

The arms 295 may be adapted for movement between collapsed and extended conditions, whereby in the collapsed condition each arm 295 is clear of the internal wall of the drill pipe and in the extended condition it is in engagement with the drill pipe for controlled positioning of the body 255 within the borehole to provide stable support for operation of the surveying instrument.

Magnetic braking may be utilised to slow the drop tool assembly 250 as it approaches the end of its descent into a borehole. The drop tool assembly 250 may for example include at least a portion of a magnetic braking system (not depicted) for slowing the drop tool assembly 250 in this manner.

The drop tool assembly 250 is provided with a means 300 for allowing a tool face of the downhole surveying instrument to be transferred to an external surface of the drop tool assembly 250. For example, the means 300 may include a tool face mark 301 on an external surface of the body 255, and one of the body 255 and a pressure barrel 302 (see FIG. 7) of the surveying instrument may include a locating lug 303 which is receivable in a groove 304 in the other of the body 255 and the pressure barrel 302 of the surveying instrument when the surveying instrument is inserted into the compartment 261 so that a tool face 305 of the instrument can thereby be synchronised with the tool face mark 301 on the external surface of the body 255. When the downhole surveying instrument comprises one or more gyroscopes, the tool face of the down hole surveying instrument may comprise a gyro tool face, and the tool face mark 301 on the external surface of the body 255 may comprise a gyro tool face mark.

The drop tool assembly may also be provided with a means 315 for allowing the tool face to be adjustably transferred to the inner tube assembly 201. For example, the means 315 may comprise a shaft (not depicted) which connects the upper portion 263 to the lower portion 265 such that the upper and lower portions 263, 265 are able to be rotated relative to each other. Rotation of the lower portion 265 relative to the upper portion 263 allows the position of a tool face mark 316 on the lower portion 265 to be adjusted relative to the tool face mark 301 and a tool face mark 227 on the inner tube assembly 201. Once the position of the tool face mark 316 has been adjusted, two nuts 318 on the shaft can be tightened to mechanically lock the portions 263, 265 together so as to prevent relative rotation between the portions 263, 265. Where the tool face mark 301 on the external surface of the body 255 comprises a gyro tool face mark, the tool face marks 316, 227 also comprise gyro tool face marks.

The downhole assembly 251 further comprises a release system 320 for allowing the drop tool assembly 250 to be released from the inner tube assembly 201 while the drop tool assembly 250 and the inner tube assembly 201 are located downhole. The release system 320 is operable to disengage the dogs 282 of the latching mechanism 281 from the spearhead point 101.

Additionally, the downhole assembly 251 comprises a synchronisation system 325 for allowing the drop tool assembly 250 to be connected to the inner tube assembly 201 so that a predetermined tool face 301 of the drop tool assembly 250 is synchronised to the inner tube assembly 201.

The synchronisation system 325 comprises a profiled portion 326 of the lower body portion 265 of the drop tool assembly 250, and a profiled portion 327 of the inner tube assembly 201. The profiled portion 326 comprises a mule shoe 328, and the profiled portion 327 comprises a mule shoe 329. The profiled portions 326, 327 are configured to engage with each other such that the drop tool assembly 250 is able to rotate relative to the inner tube assembly 201 into a home position under its own weight or with only minimal thrust.

Downhole assembly 251 also comprises a locking system 245 for mechanically locking the inner tube assembly 201 to inturn synchronise to the tool face 301 of the drop tool assembly 250. The locking system 245 may comprise a flow clutch which is provided as part of the inner tube assembly 201 and which is able to engage and disengage the connection between two co-linear components of a rock drilling device.

Rather than including the mechanical locking system 245, the tool face position of the inner tube assembly 201 may be synchronised to the drop tool assembly 250 by wireless transmission.

The drop tool assembly 250 also includes a water pressure activation system 330 for indicating when the drop tool assembly 250 has landed in or otherwise reached its final position within a borehole. The water pressure activation system 330 is operable to activate/deactivate the downhole surveying instrument. With reference to FIG. 8, the water pressure activation system 330 comprises a ball 331 that will indicate a spike in fluid pressure when pumped through a plastic tube 332 once landed. In addition, it includes a V packer 333, and fluid ports 334. Similar water pressure activation systems are known in the art, consequently the system 330 will not be further described here.

The drop tool assembly 250 and the inner tube assembly 201 may be picked up by an overshot assembly at the same time. There is the options of only using the drop tool assembly 250 to take a borehole survey when such a survey is required as it may be that a gyro survey is not required each time a core sample is obtained. In such a situation, the drop tool assembly 250 may not be attached to the inner tube assembly 201.

It should be appreciated that the scope of the invention is not limited to the scope of the preferred embodiments described.

Further, modifications and improvements can be made without affecting the scope of the invention.

Throughout this specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. 

1. Apparatus for use in a downhole survey in conjunction with core sampling, the apparatus comprising a body adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the apparatus and operated in the borehole.
 2. The apparatus defined by claim 1, wherein the downhole surveying instrument comprises a downhole survey tool or a component thereof.
 3. The apparatus defined by claim 2, wherein the downhole tool comprises one or more sensor devices selected from a group of sensor devices comprising an: orthogonal accelerometer; magnetometer; gyroscope; and a MEMS (microelectromechanical) gyro sensor.
 4. The apparatus defined by any one of the preceding claims, wherein the body, or at least relevant parts thereof, are made of material or materials which do not interfere magnetically with downhole surveying instrument.
 5. The apparatus defined by claim 4, wherein the body, or at least relevant parts thereof, are made of material which is non-magnetic.
 6. The apparatus defined by any one of the preceding claims, wherein the body incorporates a cavity for accommodating the downhole surveying instrument.
 7. The apparatus defined by any one of the preceding claims, wherein the body is configured to facilitate installation of the downhole surveying instrument in the cavity.
 8. The apparatus defined by claim 7, wherein the body is constructed in two or more parts, with one part being selectively separable from another part to provide access to the cavity.
 9. The apparatus defined by any one of the preceding claims, wherein the body is adapted to provide cushioning to afford some impact protection for the surveying instrument.
 10. The apparatus defined by claim 9, wherein the body is configured to cushion impact forces following descent into a borehole.
 11. The apparatus defined by claim 9 or 10, wherein the cushioning is provided by a cushioning mechanism incorporated in the body.
 12. The apparatus defined by claim 11, wherein the cushioning mechanism comprises an elastic structure for absorbing a shock impact.
 13. The apparatus defined by claim 12, wherein the elastic structure comprises a spring.
 14. The apparatus defined by claim 12 or 13, wherein the cushioning mechanism further comprises a shock absorber for damping the spring oscillations.
 15. The apparatus defined by any one of the preceding claims, wherein the apparatus further comprises at least a portion of a magnetic braking system.
 16. The apparatus defined by any one of the preceding claims, wherein the apparatus further comprises provision for controlled positioning thereof within the borehole for operation of the surveying instrument.
 17. The apparatus defined by claim 16, wherein the provision for controlled positioning within the borehole for operation of the surveying instrument comprises means for engaging the surrounding portion of a drill string to provide stabilization with respect to the drill string.
 18. The apparatus defined by claim 17, wherein the engaging means may be adapted to support the apparatus within the surrounding portion of the drill string in a circumferentially centred manner.
 19. The apparatus defined by 18, wherein the engaging means comprises radially disposed arms biased outwardly to contact sides of the drill string, thus centrally positioning the apparatus within the surrounding portion of the drill string.
 20. The apparatus defined by claim 19, wherein the arms each include an outer contact portion configured for contact with the inner wall of the surrounding portion of the drill string.
 21. The apparatus defined by any one of claims 17 to 20, wherein the engaging means is configured to engage the drill string for movement therealong as the apparatus is raised or lowered within the borehole.
 22. The apparatus defined by any one of claims 17 to 20, wherein the engaging means is adapted for movement between collapsed and extended conditions, whereby in the collapsed condition it is clear of the internal wall of the drill string for movement therealong and in the extended condition it is in engagement with the drill string for controlled positioning of the apparatus within the borehole to provide stable support for operation of the surveying instrument.
 23. The apparatus defined by any one of claims 17 to 22, wherein the apparatus further comprises actuation means for actuating the engaging means.
 24. The apparatus defined by any one of the preceding claims, wherein the apparatus is adapted to initiate operation of the downhole surveying instrument.
 25. The apparatus defined by any one of the preceding claims, wherein the apparatus is selected from a group of apparatus comprising: an inner tube assembly of a core drill; an overshot assembly; or an arrangement such as a sub or a drop tool assembly adapted for attachment to an inner tube assembly and/or to an overshot assembly.
 26. An inner tube assembly comprising a body having an upper portion adapted for connection to a retrieval system and a lower portion adapted to receive a core sample during a drilling operation, the body being adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the inner tube assembly and operated in the borehole.
 27. The inner tube assembly defined by claim 26, wherein the retrieval system comprises an overshot assembly attached to the end of a wireline.
 28. The inner tube assembly defined by claim 26 or 27, wherein the overshot assembly incorporates means operable for controlled positioning of the body within the borehole for operation of the surveying instrument.
 29. The inner tube assembly defined by claim 28, wherein the means operable for controlled positioning of the body within the borehole comprises means for controlling positioning of the overshot assembly within the borehole.
 30. The inner tube assembly defined by claim 29, wherein the means operable for controlled positioning of the body within the borehole comprises means for engaging the surrounding portion of a drill string to stabilise the overshot assembly with respect to the drill string.
 31. The inner tube assembly defined by claim 30, wherein the engaging means is adapted to support the overshot assembly within the surrounding portion of the drill string in a circumferentially centred manner.
 32. The inner tube assembly defined by claim 30 or 31, wherein the engaging means comprises radially disposed arms biased outwardly to contact sides of the drill string, thus centrally positioning the overshot assembly within the surrounding portion of the drill string.
 33. The inner tube assembly defined by claim 32, wherein the arms each include an outer contact portion configured for contact with the inner wall of the surrounding portion of the drill string.
 34. The inner tube assembly defined by any one of claims 30 to 33, wherein the engaging means is configured to engage the drill string for movement therealong as the overshot assembly is raised or lowered within the borehole.
 35. The inner tube assembly defined by claim 34, wherein the engaging means comprises a fixed structure configured as a carriage for movement along the internal wall of the drill string.
 36. The inner tube assembly defined by any one of claims 30 to 33, wherein the engaging means is adapted for movement between collapsed and extended conditions, whereby in the collapsed condition it is clear of the internal wall of the drill string for movement therealong and in the extended condition it is in engagement with the drill string for controlled positioning of the overshot assembly within the borehole to provide stable support for operation of the surveying instrument.
 37. An overshot assembly comprising a body having an upper portion adapted for connection to a wireline and a lower portion adapted to be releasably connected to an inner tube assembly, the body being adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the overshot assembly and operated in the borehole.
 38. The overshot assembly defined by claim 37, wherein the downhole surveying instrument comprises a downhole survey tool or a component thereof.
 39. The overshot assembly defined by claim 38, wherein the downhole tool comprises one or more sensor devices selected from a group of sensor devices comprising an orthogonal accelerometer; magnetometer; gyroscope; and a MEMS (microelectromechanical) gyro sensor.
 40. The overshot assembly defined by any one of claims 37 to 39, wherein the body, or at least relevant parts thereof, are made of material or materials which do not interfere magnetically with downhole surveying instrument.
 41. The overshot assembly defined by claim 40, wherein the body, or at least relevant parts thereof, are made of material which is non-magnetic.
 42. The overshot assembly defined by any one of claims 37 to 41 wherein the downhole surveying instrument is operable within the borehole while the overshot assembly is connected to the inner tube assembly.
 43. The overshot assembly defined by any one of claims 37 to 42 further comprising means operable for controlled positioning of the body within the borehole for operation of the surveying instrument.
 44. The overshot assembly defined by claim 43 wherein the means operable for controlled positioning of the body within the borehole comprises means for engaging the surrounding drill string to stabilise the body.
 45. The overshot assembly defined by claim 44 wherein the engaging means is adapted to support the body within the drill string in a circumferentially centred manner.
 46. The overshot assembly defined by claim 45 wherein the engaging means is configured as a centraliser.
 47. The overshot assembly defined by claim 45 or 46, wherein the engaging means comprise radially disposed arms biased outwardly to contact sides of the drill string, thus centrally positioning the body within the drill string.
 48. The overshot assembly defined by claim 47, wherein the arms each include an outer contact portion configured for contact with the inner wall of the surrounding drill string.
 49. The overshot assembly defined by any one of claims 44 to 48, wherein the engaging means is configured to engage the drill string for movement therealong as the overeshot assembly is raised or lowered within the borehole
 50. The overshot assembly defined by claim 49, wherein the engaging means comprises a fixed structure configured as a carriage for movement along the internal wall of the drill string.
 51. The overshot assembly defined by any one of claims 44 to 48, wherein the engaging means is adapted for movement between collapsed and extended conditions, whereby in the collapsed condition it is clear of the internal wall of the drill string for movement therealong and in the extended condition it is in engagement with the drill string for controlled positioning of the body within the borehole to provide stable support for operation of the survey instrument.
 52. The overshot assembly defined by any one of claims 37 to 51, wherein the body incorporates a cavity for accommodating the downhole instrument.
 53. The overshot assembly defined by claim 52, wherein the body is configured to facilitate installation of the downhole surveying instrument in the cavity.
 54. The overshot assembly defined by claim 53, wherein the body comprises two or more parts, with one part being selectively separable from another part to provide access to the compartment.
 55. The overshot assembly defined by any one of claims 37 to 54, wherein the body is adapted to provide cushioning to afford some impact protection for the surveying instrument.
 56. The overshot assembly defined by claim 55, wherein the body is configured to cushion impact forces when moving into contact with a downhole inner tube assembly following descent on the wireline.
 57. The overshot assembly defined by claim 55 or 56, wherein the cushioning is provided by a cushioning mechanism incorporated in the body.
 58. The overshot assembly defined by claim 57, wherein the cushioning mechanism comprises an elastic structure for absorbing a shock impact.
 59. The overshot assembly defined by claim 58, wherein the elastic structure comprises a spring.
 60. The overshot assembly defined by claim 58 or 59, wherein the cushioning mechanism further comprises a shock absorber for damping spring oscillations.
 61. The overshot assembly according to any one of claims 57 to 60, wherein the cushioning mechanism comprises a parachute or other controlled descent system or method.
 62. The overshot assembly defined by any one of claims 57 to 61, wherein the cushioning mechanism is incorporated within the body between two parts thereof.
 63. The overshot assembly defined by any one of claims 37 to 62, wherein the overshot assembly further comprises at least a portion of a magnetic braking system for slowing the overshot assembly.
 64. The overshot assembly defined by any one of claims 37 to 63, wherein the lower portion of the body incorporates a latching mechanism for releasable connection to a mating formation on the inner core tube assembly.
 65. The overshot assembly defined by claim 64, wherein the body incorporates an actuator for actuating the latching mechanism to engage/disengage the formation upon engagement between the overshot assembly and the inner core tube assembly.
 66. The overshot assembly defined by any one of claims 37 to 65, wherein the overshot assembly is provided with a means for allowing a tool face of the downhole surveying instrument to be transferred to an external surface of the overshot assembly.
 67. The overshot assembly defined by claim 66, wherein the overshot assembly is provided with a means for allowing the tool face to be transferred to the inner tube assembly.
 68. Apparatus adapted for connection to an inner tube assembly, the apparatus comprising a body being adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the inner tube assembly and operated in the borehole.
 69. A drop tool assembly comprising a body having an upper portion adapted to be releasably connected to a retrieval system and a lower portion adapted to be releasably connected to an inner tube assembly, the body being adapted to receive a downhole surveying instrument, whereby the downhole surveying instrument can be conveyed along a borehole with the drop tool assembly and operated in the borehole.
 70. The drop tool assembly defined by claim 69, wherein the retrieval system comprises an overshot assembly attached to the end of a wireline.
 71. The drop tool assembly defined by claim 69 or 70, wherein the drop tool assembly incorporates means operable for controlled positioning of the body within the borehole for operation of the surveying instrument.
 72. The drop tool assembly defined by claim 71, wherein the means operable for controlled positioning of the body within the borehole may comprises means for engaging the surrounding drill string to stabilise the body.
 73. The drop tool assembly defined by claim 72, wherein the engaging means is adapted to support the body within the drill string in a circumferentially centred manner.
 74. The drop tool assembly defined by claim 73, wherein the engaging means is configured as a centraliser.
 75. The drop tool assembly defined by any one of claims 72 to 74, wherein the engaging means comprises radially disposed arms biased outwardly to contact sides of the drill string, thus centrally positioning the body within the drill string.
 76. The drop tool assembly defined by claim 75, wherein the arms each include an outer contact portion configured for contact with the inner wall of the surrounding drill string.
 77. The drop tool assembly defined by any one of claims 72 to 76, wherein the engaging means is configured, to engage the drill string for movement therealong as the drop tool assembly is raised or lowered within the borehole.
 78. The drop tool assembly defined by claim 77, wherein the engaging means comprises a fixed structure configured as a carriage for movement along the internal was of the drill string.
 79. The drop tool assembly defined by any one of claims 72 to 76, wherein the engaging means is adapted for movement between collapsed and extended conditions, whereby in the collapsed condition it is clear of the internal wall of the drill string for movement therealong and in the extended condition it is in engagement with the drill string for controlled positioning of the body within the borehole to provide stable support for operation of the surveying instrument.
 80. The drop tool assembly defined by any one of claims 69 to 79, wherein the body incorporates a cavity for accommodating the downhole surveying instrument.
 81. The drop tool assembly defined by claim 80, wherein the body is configured to facilitate installation of the downhole surveying instrument in the cavity.
 82. The drop tool assembly defined by claim 81, wherein the body is constructed in two or more parts, with one part being selectively separable from another part to provide access to the cavity.
 83. The drop tool assembly defined by any one of claims 69 to 82, wherein the drop tool assembly further comprises at least a portion of a magnetic braking system.
 84. The drop tool assembly defined by any one of claims 69 to 83, wherein the body has an upper portion adapted to be releasably connected to an overshot assembly, and a lower portion adapted to be releasably connected to an inner tube assembly.
 85. The drop tool assembly defined by claim 84, wherein the lower portion of the body incorporates a latching mechanism for releasable connection to a mating formation on the inner tube assembly.
 86. The drop tool assembly defined by claim 85, wherein the body incorporates an actuator for actuating the latching mechanism to engage the formation upon engagement between the drop tool assembly and the inner core tube.
 87. The drop tool assembly defined by any one of claims 84 to 86, wherein the upper portion of the body incorporates a mating formation.
 88. The drop tool assembly defined by any one of claims 69 to 87, wherein the downhole surveying instrument comprises a downhole surveying tool or a component thereof.
 89. The drop tool assembly defined by claim 88, wherein the downhole tool comprises one or more sensor devices selected from a group of sensor devices comprising an: orthogonal accelerometer; magnetometer; gyroscope; and a MEMS (microelectromechanical) gyro sensor.
 90. The drop tool assembly defined by any one of claims 69 to 89, wherein the body, or at least relevant parts thereof, is made of material or materials which do not interfere magnetically with the geo-magnetic device.
 91. The drop tool assembly defined by claim 90, wherein the body or at least relevant parts thereof are made of material which is non-magnetic.
 92. The drop tool assembly defined by any one of claims 69 to 91, wherein the downhole surveying instrument is operable within the borehole either while the drop tool assembly is connected to the inner tube assembly, or when the drop tool assembly is not connected to the inner tube assembly.
 93. The drop tool assembly defined by any one of claims 69 to 92, wherein the drop tool assembly is provided with a means for allowing a tool face of the downhole surveying instrument to be effectively transferred to an external surface of the drop tool assembly.
 94. The drop tool assembly defined by claim 93, wherein the drop tool assembly is provided with a means for allowing the tool face to be adjustably transferred to the inner tube assembly.
 95. The drop tool assembly defined by any one of claims 69 to 92, wherein the drop tool assembly also comprises a water pressure activation system for indicating when the drop tool assembly has landed or otherwise reached its final position within a borehole.
 96. The drop tool assembly defined by claim 95, wherein the water pressure activation system is operable to activate/deactivate the downhole surveying instrument.
 97. A downhole assembly comprising an inner tube assembly according to any one of claims 26 to 36 in combination with an overshot assembly.
 98. The downhole assembly defined by claim 97, wherein the downhole assembly further comprises a release system for allowing the overshot assembly to be released from the inner tube assembly while the overshot assembly and the inner tube assembly are located downhole.
 99. The downhole assembly defined by claim 97 or 98, wherein the downhole assembly also comprises a synchronisation system for allowing the overshot assembly to be connected to the inner tube assembly so that a predetermined tool face of the overshot assembly is synchronised to the inner tube assembly.
 100. The downhole assembly defined by claim 99, wherein the synchronisation system comprises a profiled portion of the overshot assembly and a profiled portion of the inner tube assembly, the profiled portions being configured to engage with each other such that the overshot assembly is able to rotate relative to the inner tube assembly into a home position under its own weight or with minimal thrust.
 101. The downhole assembly defined by claim 100, wherein each profiled portion comprises a respective mule shoe.
 102. The downhole assembly defined by any one of claims 97 to 101, wherein the downhole assembly further comprises a locking system for mechanically locking the inner tube assembly to inturn synchronise to a tool face of the overshot assembly.
 103. The downhole assembly defined by claim 102, wherein the locking system comprises a flow clutch.
 104. The downhole assembly defined by any one of claims 97 to 101, wherein a tool face position of the inner tube assembly is able to be synchronised to the overshot assembly by wireless transmission.
 105. The downhole assembly defined by any one of claims 97 to 104, wherein the overshot assembly comprises means operable for controlled positioning of the body within the borehole for operation of the surveying instrument.
 106. A downhole assembly comprising a drop tool assembly defined by any one of claims 69 to 96 in combination with at least one of an inner tube assembly and an overshot assembly.
 107. The downhole assembly defined by claim 106, wherein the downhole assembly also comprises a release system for allowing the drop tool assembly to be released form the inner tube assembly while the drop tool assembly is located down hole.
 108. The downhole assembly defined by claim 106 or 107, wherein the downhole assembly also comprises a synchronisation system for allowing the drop tool assembly to be connected to the inner tube assembly so that a predetermined tool face of the drop tool assembly is synchronised to the inner tube assembly.
 109. The downhole assembly defined by claim 108, wherein the synchronisation system comprises a profiled portion of the drop tool assembly and a profiled portion of the inner tube assembly, the profiled portions being configured to engage with each other such that the drop tool assembly is able to rotate relative to the inner tube assembly into a home position under its own weight or with minimal thrust.
 110. The downhole assembly defined by claim 109, wherein each profiled portion comprises a respective mule shoe.
 111. The downhole assembly defined by any one of claims 106 to 110, wherein the downhole assembly further comprises a locking system for mechanically locking the inner tube assembly to inturn synchronise to a tool face of the drop tool assembly.
 112. The downhole assembly defined by claim 111, wherein the locking system comprises a flow clutch.
 113. The downhole assembly defined by any one of claims 106 to 110, wherein a tool face position of the inner tube assembly is able to be synchronised to the drop tool assembly by wireless transmission.
 114. A method of conducting a borehole surveying operation using a surveying instrument accommodated in an apparatus defined by any one of claims 1 to
 25. 115. A method of conducting a borehole surveying operation using a surveying instrument accommodated in an inner tube assembly defined by any one of claims 26 to
 36. 116. A method of conducting a borehole surveying operation using a surveying instrument accommodated in an overshot assembly defined by any one of claims 37 to
 67. 117. A method of conducting a borehole surveying operation using a surveying instrument accommodated in an apparatus defined by claim
 68. 118. A method of conducting a borehole surveying operation using a surveying instrument accommodated in a drop tool assembly defined by any one of claims 69 to
 96. 119. A method of conducting a borehole surveying operation using a surveying instrument accommodated in an inner tube assembly forming part of a combination defined by any one of claims 97 to
 105. 120. A method of conducting a borehole surveying operation using a surveying instrument accommodated in a drop tool assembly forming part of a combination defined by any one of claims 106 to
 113. 121. A method of conducting a borehole surveying operation using apparatus incorporating an onboard downhole surveying instrument, the method comprising lowering the apparatus down the borehole, taking a measurement down the borehole using the onboard downhole surveying instrument, and retrieving a core sample using the apparatus.
 122. A method of conducting a borehole surveying operation using an inner tube assembly incorporating an onboard downhole surveying instrument, the method comprising deploying the inner tube assembly in the borehole, taking a measurement down the borehole using the onboard downhole surveying instrument, and retrieving the inner tube assembly containing a core sample.
 123. A method of conducting a borehole surveying operation using an overshot defined by any one of claims 37 to
 67. 124. A method of conducting a borehole surveying operation using an overshot assembly incorporating an onboard downhole surveying instrument, the method comprising lowering the overshot assembly down the borehole, taking a measurement down the borehole using the onboard downhole surveying instrument, and retrieving an inner tube assembly containing a core sample using the overshot assembly.
 125. A method of conducting a borehole surveying operation using a drop tool assembly incorporating an onboard downhole surveying instrument, the method comprising lowering the drop tool assembly down the borehole using the onboard downhole surveying instrument, and retrieving an inner tube assembly containing a core sample using the drop tool assembly and an overshot assembly. 